1. Field of the Invention
The present invention relates to timing recovery in a modem receiver and, in particular, to fine-timing recovery for a quadrature amplitude modulation (QAM) modem receiver.
2. Discussion of the Prior Art
The basic function of any communications system is to transmit information over a communication channel from an information source to a destination as fast and as accurately as possible.
There are two general types of information sources. Analog sources, such as a telephone microphone, generate a continuous signal. Digital sources, such as a digital data processing system, generate a signal that consists of a sequence of pulses.
Communications channels that are designed to transmit analog signals (e.g., the telephone network) have characteristics which make it difficult for them to transmit digital signals. To permit the transmission of digital pulse streams over an analog channel, it is necessary to utilize the digital data pulses to modulate a carrier waveform that is compatible with the analog transmission channel.
The equipment that performs the required modulation is generally referred to as a "MODEM". The term "MODEM" is an acronym for MOdulator-DEModulator, since one piece of equipment typically includes the capability not only to modulate transmitted signals, but also to demodulate received signals to recover the digital data from the modulated analog carrier waveform.
While passing through the transmission channel, the modulated carrier waveform suffers from distortion introduced both by the system itself an by noise contamination. Thus, one of the tasks of the modem's demodulating receiver function is to filter the signal received from the transmission channel to improve the signal-to-noise ratio. The modem receiver also recovers timing information from the received signal to provide sampling points for recovering the digital data. The modem receiver may also condition the data in other ways to make it suitable for additional processing.
In a conventional modem, the signal filtering, timing recovery and conditioning tasks are performed by three functional units: analog-to-digital conversion circuitry ("analog front end") that converts the received modulated carrier waveform to a digitized replica, a digital signal processor (DSP) that retrieves the digital data from the digitized replica using a recovered timing signal, and a control function for controlling both the analog front end and the DSP. The DSP recovers the data by implementing a signal conditioning and data recovery algorithm that is specific to the type of data being received.
For example, the DSP function in a facsimile (fax) machine modem implements a special purpose algorithm that can only be used for recovering digital fax data. In the case of a fax system, the data to be recovered is a digital bit map that corresponds to the transmitted hard copy image and which has been compressed to facilitate efficient transmission. The algorithm implemented by the digital signal processor function of the receiving fax machine's modem is a dedicated "fax" algorithm that has been designed specifically for accurately recovering the compressed bit map. It cannot recover digital data in a format other than a compressed bit map, e.g. voice mail data or data modem applications. A different digital signal processor implementing a different dedicated "voice mail" or "data modem" algorithm is needed for each of these other applications.
As shown in FIG. 1, a conventional fax machine architecture may be partitioned into two major functional blocks: a special purpose fax modem block 1 of the type described above for recovering a compressed bit map from a modulated carrier waveform and a general purpose processor block 2 for performing those tasks required to convert the compressed bit map to a corresponding hard copy image.
Quadrature amplitude modulation, or QAM is the independent amplitude modulation of two orthogonal channels using the same carrier frequency.
Fine-timing recovery is a critical operation in a QAM modem. Previous QAM timing recovery methods require significant CPU power to implement in a software modem. In addition, the long feedback delays inherent in conventional QAM modems are difficult to tune and can lead to unstable operation.